Electrolytic reduction of aluminum bromide



April 1945' w. G. LOVELL ET AL 2,373,320

ELECTROLYTIC REDUCTION OF ALUMINUM BROMIDE Filed Nov. 12, 1940Zinventors I I 9 t i y Gttornegs Patented Apr. 10, 1945 UNITED STATESPATENT OFFICE ELECTROLYTIC REDUlvgl'lggN F ALUMINUM Wheeler G. Lovelland Nelson E. Phillips, Detroit, Mich., assignors to General MotorsCorporation, Detroit, Mich., a corporation of Delaware ApplicationNovember 12, 1940, Serial No. 365,188

9 Claims.

that has more advantageous energy relations than present methods, toprovide a process and apparatus for producingaluminum that reduces thecost as compared with present practice, to provide a practical andeconomical method and apparatus for producing aluminum of a high degreeof purity; to provide improvements in a method and apparatus ofproducing substantially pure aluminum which makes it possible to utilizeeconomically a wider variety of cheaper aluminum containing rawmaterials than present commercial processes; to provide improvements ina method and apparatus for recovering substantially pure aluminum as itis formed by the electrolytic reduction of the aluminum compound in theelectrolyte; to provide improvements in.

anodes and cathodes, especially those for use in by the flowingelectrolyte, special addition agents may be provided for theelectrolyte. In order to make the electrolyte suiiiciently conducting,ingredients such as sodium bromide or potassium bromide form a portionof the electrolyte. In the device illustrated the aluminum particles arefiltered from the electrolyte by .means of a filter type piston, butother methods for separating the solid and liquid may beusedu Detaileddescription In the drawing is a reservoir l0 adapted to contain theelectrolyte. A suitable heating means l2 prpvides sufficient heat tomake up for heat lost from the apparatus and to maintain the electrolytein the chamber at-therequired temperature necessary to keep theelectrolyte in a liquid state when the apparatus is not being operated.Makeup electrolyte may be added to the reservoir from time to time bymeans of opening i4 and cover l6. Electrolyte is pumped from thereservoir by any suitable pump, which in the form shown is constructedas follows. Extending into the electrolyte in the reservoir is a glasscylinder ill, the

2 lower end of which is in flow communication the electrolytic reductionof aluminum bromide. I Other objects and advantages of the inventionwill become more apparent as the description proceeds. Reference isherewith made to the accompanying drawing forming a portion of thisspecification in which the figure illustrates somewhat diagrammaticallyone form of apparatus adapted to carry out the method in accordance withthe invention.

General description The device illustrated comprises a means for heatingand continuously circulating through a generally closed system a fusedelectrolyte of suitable boiling point containing aluminum bromide, airand water being excluded from the system. The fused, non-aqueouselectrolyte passes through an anode which directs the electrolyte inproper relation with respect to a vibra-. tory cathode. The device is soconstructed and controlled as to deposit the aluminum at the cathode ina finely divided form and to remove moved by vibrating the cathode orreadily carried with the electrolyte by means of openings 20. In thecylinder adjacent the openings is an impeller 22 formed of tantalum. Theimpeller is secured to a glass rod 24 which is adapted to be rapidlyrotated by the electric motor 26. Rotation of the impeller duringoperation thus causes a fiow of the electrolyte continuously throughcylinder Hi to an outlet 28, thence to the electrolytic cell proper,indicated generally by the reference numeral 30. 3

The electrolyte enters the cell and passes through an electricallyconducting cup-shaped anode32 having a series of openings 34 in thelower end thereof. Preferably the anode is formed of carbon. Otherinsoluble conducting anodes may be used. The openings are so arranged asto direct the electrolyte downwardly toward a cathode 36. y

In the form shown the cathode is a rod of tantalum curved to form aring, the ring being joined electrolyte are desirable.

tends to deposit in the form of feathery rees" or dendrites andadvantage is taken of this. Certain addition agents may be added to theelectrolyte to control more fully the deposition and cause the formationof the aluminum in finely divided form.

' The source of electric current is connected in any desired manner tothe anode and cathode. The positive side of the current source may becon- I nected to a flange 31 of the anode, for example,

whilethe other side of the current source may be connected to arm llconnected to the cathode.

The aluminum particles fiow with the circulating electrolyte from theelectrolytic cell through the' outlet 56 and through the tube 5] to afilter chamber indicated generally by the reference numeral 60. Withinthe filter chamber is a piston element 82 of carbon or other suitablematerial.

said\ element having openings 84 in the head thereof. Above the head ofthe piston is-a filter element 68. In the device illustrated the filterelement is a glass fabric. The purpose of the glass-filter and theopenings in the head of the piston is to separate the aluminum particlesfrom moves the cake from the active filter area so as to permit betterflow and also to keep the "cake" inlet tube 88. Most of the vaporizedaluminum bromide leaving the cell is condensed by the condenser-refluxcolumn and flows back into the cell.

' an outlet 80 arranged near the top thereof, and

away from the electrolyte which may contain small amounts of bromine andmight thus otherwise dissolve some of the metal. Mechanical means may beused for suspending the cake from piston 10. The cake" of metal isgradually built up by successive additions andpreferably extends abovethe level of the electrolyte. thus permitting the electrolyte to drainoff. The piston It may be removed from time to time by means of atubular glass rod 12 secured thereto, a cover ll of the filter chamberbeing removed during the removal of the piston I0, and all ora portionof the cake of aluminum may be removed from the piston 10,

The strained electrolyte leaves the filter chamber through an outlet I5and returns to the heated reservoir In by means of tube It. In the formof apparatus illustrated, the pump 22, etc.. moves the electrolyte tothe cup-shaped anode and the flow of electrolyte from this point to thereservoir l0 isdue to the force of gravity acting thereon.

As the aluminum is set free at the vibratine 1 cathode, bromine isliberated at the anode, which is centrally arranged in the upper portionof the cell. The free bromine boils up through the hot electrolyte. Itis importantto maintain high temperatures so that the bromine will notbe too soluble in the electrolyte and temperatures appreaching theboiling point of the bromine-free 7 Under these conditions the aluminumhalide from the electrolyte will boil off with the bromine, and to makea partial separation of the two and return the aluminum bromide to thecell, the vapors pass through a reflux column. This comprises a glasstube or receptacle 80 nearly filled with glass rings or beads 82. Thetop of receptacle 80 is closed by a glass cover 8|. Surrounding aportion of receptacle 80 is an air chamber 84 having an airoutlet 86 andan air by means of a passage 92 enter a glass receiver 8!. A watercondenser 96 condenses any uncondensed bromine and aluminum bromidevapors and returns the-same to the receiver.

The design of the anode is important. The bromine must be evolvedrapidly in order to avoid the formation of a gaseous film whichincreases the'electrical resistance of the cell. The anode must bedesigned to be as close to the cathode as possible in order to keep thecell resistance at a minimum. Some circulation of electrolyte past theanode is necessary in order to avoid the formation of a layer of lowconductivity; too much stirring is to be avoided as it promotes thesolution of the bromine in the electrolyte with theconsequentredissolving ofthe aluminum. The electrical resistance of theanode should be as low as possible. Several forms of anodes have beenused in accordance with the above requirements, the form shown in thedrawing being preferred. The bottom of the anode i turned to a radius sothat it will flt within the cathode and it has a streamlined surface forthe free removal of the bromine.

It is important in the operation of the cell that the surface of theanode not be allowed to become a poor conductor of electricity. Undercertain conditions, particularly after moisture has had access to theelectrolyte, a high resistance film may be built up on the anode. Forthis reason, the electrolyte must at all times be protected from theintroduction of even small amounts of water, in order to assurecontinuous satisfactory operation of the cell for long periods of time.

The requirements for the cathode are that it should be a good conductorand so shaped that the flow of electric current will be equallydistributed and so that it will not obstruct the flow of liquid or ofsolid metal particles. High curmetal will be in a form which can bereadily dislodged. The upper limit of electrical current in anyparticular application i governed by the amount of heat generated andthis is dependent upon the electrical resistance of the cell. The amountof heat generated should not be such as to cause too vigorous boiling ofthe electrolyte. Another factor to be considered in the. determinationof the maximum current is the energy efiiciency desired. The electricalenergy lost in the internal resistance of the cell as heat, varies withthe square of the current, while the amount of metal produced variesdirectly as the current. The current used in practice in the celldepends consequently upon the economic balance between pounds of metalproduced per hour, and kilowatt hours per pound required to produce it.

In one form and size of apparatus constructed in accordance with theinvention from 50 to 225 amperes of current have been passed through theelectrolyte in a cell having a diameter of .six inches. Currentdensities as high as approximately l5,000 amperes per square foot ofcathode area have been used. Good current efficiencies in the range ofto have been consistently obtained.

a matter of experience and compromise that the sodium bromide (when thisis used as the current carrier) be about 24% of the electrolyte, thebalance being aluminum bromide. Lower proportions of sodium bromideincrease the resistance of the electrolyte, while higher proportionsreduce the resistance. Considering the electrical resistance alone, theideal would be the largest possible amount of sodium bromide. Thetemperature of the electrolyte preferably should be maintained as nearthe boiling point of the electrolyte as can be in order to remove thebromine as rapidly as possible from the vicinity of the electrode and tokeep the electrolyte as free of bromine as possible. In view of this,the lower the concentration of sodium bromide the lower the permissibleoperating temperature, while the higher the concentration or sodiumbromide the higher the required operating temperature. With the atpresent preferred bath composition (24% sodium bromide and 76% aluminumbromide) an electrolyte temperature of about 850 F. has been used withsuccess. With an electrolyte composed of 10% sodium bromide and 90%aluminum bromide an operating temperature of about 500 F. may be used,while with an electrolyte composed of sodium bromide and 80% aluminumbromide an operating temperature of 650 F. may be used.

In order to ensure that the electrolyte flow through the cell in as nearstreamline flow as possible, the anode is so designed as to distributethe electrolyte substantially uniformly over the cross section of thecell. In the form illustrated the aluminum released from the cathodeflows downwards with the electrolyte. The rate of flow of theelectrolyte should be fast enough so a to continuously remove thealuminum from the cell and still not agitate the bath enough to causethe bromine to redissolve any appreciable amount of the aluminum.

It has been found desirable to add small amounts of certain substancesto the bath or electrolyte in order to control the size of the particlesof aluminum. Under some conditions of operation, especially with verypure materials, or on prolonged electrolysis of less pure materials, thealuminum forms moss-like aggregations, which, when detached from thecathode, tend to agglomerate in large pieces of the mossy, porous typeup to an inch or more in diameter. Such large pieces are undesirable inthat they do not flow through the cell properly since they are subjectedto diverse liquid currents and to mechanical striking by the vibratorycathode. As a result they may be subjected to the action of bromine inthe cell and partially dissolved, thus resulting in a decrease in thecurrent efllciency of the cell. The large pieces also have a tendency toclog the connecting tubes to the filter chamber. We have found thatsmall amounts of. such materials as naphthalene, phena-nthrene, oranthracene when added to the electrolyte prevents the formation of thelarge particles of aluminum and thu obviates the above mentioneddisadvantages of such large particles. One part of the addition agent inabout a quarter of amillionparts of electrolyte has proven sufflcient.The addition agent must be replenished from time to time as needed.Other addition agents which have been used are lubricating oil, rubberand the like. Although we do not wish to be bound by any definitetheory, we believe that the mechanism tion agent is added to the hotelectrolyte, it decompose and forms insoluble colloidal carbon which isabsorbed on the clean and'active surfaces 0! the aluminum particles soas to keep the particles from sticking or welding together to form largeparticles.- In accordance with our theory organic materials whichdecompose in aluminum bromide at bath temperatures to form colloidalcarbon may be used effectively. We prefer to use hydrocarbons orcompounds containing little or no oxygen so a to prevent the formationof aluminum oxide as a final product to contaminate the bath.

Due to the fact that aluminum bromide solutions of bromine are verycorrosive to most metals, especially at the high temperatures usedherein, it is necessary to use materials that are not attacked thereby.In the form of apparatus shown, the cell proper, the filter chamber, thereservoir and the passages connecting the same are formed of Pyrexglass. It is contemplated that other vitreous and ceramic materials, aswell as enamel surfaces may be used, as well as carbon; when itselectrical properties permit. While it is at present preferred that theanode be of carbon it may be formed of tantalum. The best results havebeen obtained with a cathode formed of tantalum. It is contemplated thatthe cathode may also be formed of carbon. I The "cake" of aluminum afterremoval from the filter chamber is a coherent, spongy mass which may bereadily broken. up. It contains small amounts of aluminum bromide andsodium bromide (when sodium bromide is used as the current carrier). The"cake" of aluminum may be heated in an electric muiiie or other furnaceto melt the aluminum. During the heating the small amount of aluminumbromide is distilled off and may be recovered for return to theapparatus.

49 The sodium bromide and the aluminum melt and the latter separatesinto a lower layer which may be drained off and may be cast into pigs.Remarkably pure aluminum has been produced in accordance with the\invention.

We wish it to be understood that we do not desire to be limited to theexact details of construction and operation shown and described,

of the action is about as follows: When the addi- 15 for obviousmodifications will occur to a person skilled in the art.

We claim:

1. The process of producing aluminum which comprises, flowing a streamof a fused, non-aqueous electrolyte comprising essentially aluminumbromide and a bromide of the class consisting of sodium bromide andpotassium bromide downwardly from an anode to a vibrating cathode,passing an electric current from the anode to the cathode through theflowing stream of electrolyte While.the electrolyte is at substantiallyits boiling point to liberate aluminum at the cathode in solid,particulate form and to liberate bromine at the anode, moving theelectrolyte and the aluminum particles therein that become detached fromthe cathode to a point beyond the cathode, and separating the aluminumparticles from the electrolyte, the percentage of aluminum bromide inthe electrolyte varying from about 76 to 90.

2. A process as in claim 1 in which the electrolyte has added theretoone part in about a quarter of a million parts of electrolyte, of anaddition agent of the class consisting of naphthalene, phenanthrene andanthracene.

3. A process as in claim 1 in which the electrolyte has added theretoone part of lubricating oil in about a quarter of a million parts ofelectrolyte,

4. A process as in claim 1 in which the electrolyte has added theretoone part of rubber in about a quarter of a million parts of electrolyte.

5. The process of producing aluminum which comprises, flowing a streamof a fused, nonaqueous electrolyte comprising essentially aluminumbromide and sodium bromide downwardly from an anode to a vibratingcathode, passing an electric current from the anode to the cathodethrough the flowing stream of electrolyte while the electrolyte is atsubstantially its boiling point to liberate aluminum at the cathode insolid, particulate form and to liberate bromine at the anode, moving theelectrolyte and the aluminum particles therein that become detached fromthe cathode to a point beyond the cathode, and separating the aluminumparticles from the electrolyte, the percentage of aluminum bromide inthe electrolyte varying from. about 76 to 90.

6. A process as in claim 5 in which the electrolyte has added theretoone part in about a quarter of a million parts 01' electrolyte, of anaddition agent of the class consisting of naphthalene, phenanthrene andanthracene.

7. A process as in claim 5 in which the electrolyte has added theretoone part of lubricating oil in about a quarter of a million parts ofelectrolyte.

8. A process as in claim 5 in which the electrolyte has added theretoone part of rubber in about a quarter of a. million parts ofelectrolyte.

9. The process of producing aluminum which comprises, flowing a streamof a fused, nonaqueous electrolyte comprising essentially aluminumbromide and sodium bromide downwardly from an anode to a vibratingcathode, passing an electric current from the anode to the cathodethrough the flowing stream of electrolyte while the electrolyte isatsubstantially its boiling point to liberate aluminum at the cathode insolid, particulate form and to liberate bromine at the anode, moving theelectrolyte and the aluminum particles therein that become detached fromthe cathode to a point beyond the cathode, and separating the aluminumparticles from the electrolyte, the percentage of aluminum bromide inthe electrolyte being about 76.

WHEELER G. LOVELL. NELSON E. PHILLIPS.

